Diffusion Furnaces Employing Ultra Low Mass Transport Systems and Methods of Wafer Rapid Diffusion Processing

ABSTRACT

Multi-zone, solar cell diffusion furnaces having a plurality of radiant element (SiC) or/and high intensity IR lamp heated process zones, including baffle, ramp-up, firing, soaking and cooling zone(s). The transport of solar cell wafers, e.g., silicon, selenium, germanium or gallium-based solar cell wafers, through the furnace is implemented by use of an ultra low-mass, wafer transport system comprising laterally spaced shielded metal bands or chains carrying non-rotating alumina tubes suspended on wires between them. The wafers rest on raised circumferential standoffs spaced laterally along the alumina tubes, which reduces contamination. The bands or chains are driven synchronously at ultra-low tension by a pin drive roller or sprocket at either the inlet or outlet end of the furnace, with appropriate tensioning systems disposed in the return path. The high intensity IR flux rapidly photo-radiation conditions the wafers so that diffusion occurs &gt;3× faster than conventional high-mass thermal furnaces.

CROSS-REFERENCE TO RELATED CASE

This application is the Regular U.S. patent application corresponding toand claiming the benefit of U.S. Provisional Application Ser. No.61/170,051 filed by the same inventors on Apr. 16, 2009, entitledDiffusion Furnace Having Low Mass Band Transport System and RapidDiffusion Process Using High Intensity IR Flux, the benefit of thefiling date of which is hereby claimed under 35 US Code.

FIELD

The invention relates to continuous conveyor diffusion furnaces forprocessing solar cell wafers by use of radiant or/and IR lamp heating inthe range of 900-1100° C. to cause the diffusion of P and/or B dopantcompositions into the silicon (or other advanced material) of the waferto create a p-n junction surface layer or/and back surface field layer.More specifically, the invention is directed to a solar cell diffusionfurnace having one or more firing zones and an ultra-low mass, lowfriction, transport system, in which the wafers are supported on aluminatubes, preferably including stand-off projections to space the wafersabove the tubes. The inventive diffusion furnace with low mass transportsystem provides a rapid diffusion process cycle by use of thermalradiant heating or high intensity IR radiant flux in one or more of theprocess zones, resulting in faster diffusion processing, longertransport system life, reduced wafer contamination, faster/higherthroughput with greater yield of properly doped cells, and lower energy,operating and maintenance costs.

BACKGROUND

The fabrication of silicon based solar cells requires a number ofspecialized processes to occur in a specific order. Generally theseprocesses include single crystalline silicon ingots grown in crystalgrowing furnaces or cast into multi-crystalline blocks in “directionalsolidification” furnaces. The result of these processes are long“sausage-shaped” single crystal masses called ingots, ormulti-crystalline blocks, from which thin slices of silicon are cuttransversely with “wire saws” to form rough solar cell wafers. Thesewafers, whether made up of a single crystal or multiple crystalsconjoined together, are then processed to form smooth wafers in the 150to 330 micrometer range of thickness. Because of the scarcity ofsuitable silicon, the current trend is towards making the wafersthinner, typically 140-180 micrometers thick.

Finished raw wafers are then processed into functioning solar cells,capable of generating electricity by the photovoltaic effect. Waferprocessing starts with various cleaning and etching operations, followedby a 2-stage process called diffusion which creates a semi-conducting“p-n”, junction diode, followed by a third process in which Aluminumpaste coatings are screen printed on the wafer front and back sides andthen fired into the p-n junction or back contact layer, where they actas ohmic collectors and grounds, respectively.

The diffusion process broadly comprises two stages: a first stage ofapplying (coating) and drying one or more types of dopant materials tothe front and/or back side of the wafers, followed by a second stage ofheating (firing) the coated wafers in a diffusion furnace, chamber, orheating zone, to cause diffusion of the dopant composition into the Si(or other advanced material) wafer substrate to form the p-n junctionlayer, or back contact layer. This invention is directed to improveddiffusion furnaces and firing processes, schedules and thermal profiles.The following is a summary of the subsequent steps of processingdiffusion fired wafers, which puts the invention into context.

Diffusion occurs at high temperatures in the presence of variousphosphorous sources such as a sprayed liquid of dilute phosphoric acidor a vapor of phosphorous oxichloride (POCl₃) created by bubblingnitrogen, N₂, through liquid POCL₃. The thus-doped Si forms the“emitter” layer of the photovoltaic cell, that is, the layer that emitselectrons upon exposure to sunlight (the normal photon source). Theseelectrons are collected by a fine web of screen printed metal contactsthat are sintered into the surface of the cell by a metallizationfurnace, as mentioned above.

To enhance the ability to form low resistance screen-printed metalcontacts to the underlying silicon p-n junction emitter layer,additional amounts of phosphorus are deposited onto the front surface ofthe wafer. The phosphorous is driven into the wafer via a hightemperature diffusion process. Current processes typically last 20-30minutes. The extra “electrically active” phosphorus enables the lowresistance contacts to be formed. However, the formation of suchcontacts is at the expense of a loss in cell efficiency. The cellefficiency loss arises as a result of electron-hole pairs generated ator near the surface through the absorption of higher energy but shortwave length photons. These “blue light” photons quickly recombine andare lost, thereby eliminating their contribution to the power generationof the cell.

After diffusion and various cleaning, laser edge ablation, and etchingprocesses to remove unwanted semi-conductor junctions from the sides ofthe wafers, the wafers are coated with an anti-reflective coating,typically silicon nitride (SiN₃), generally by plasma-enhanced chemicalvapor deposition (PECVD). Between some of these processes, the wafersare dried in preparation for subsequent processes in low temperaturedrying ovens.

The SiN₃ anti-reflective coating (ARC) is deposited to a thickness ofapproximately ¼ the wavelength of light of 0.6 microns. After ARCapplication, the cells exhibit a deep blue surface color (or browncolor, depending on the coating material used). The ARC minimizes thereflection of incident photons having wavelengths around 0.6 microns.The ARC SiN_(x) coating is created in the PECVD process by mixingsilane, SiH₄, ammonia, NH₃, and pure nitrogen, N₂, gases in variousconcentrations in a high or low frequency microwave field. The hydrogendissociates and diffuses very rapidly into the silicon wafer. Thehydrogen has a serendipitous effect of repairing bulk defects,especially in multi-crystalline material. The defects are traps whereelectron-hole pairs can recombine thereby reducing cell efficiency orpower output.

During subsequent IR metallization firing, elevated temperatures (above850° C.) will cause the hydrogen to diffuse back out of the wafer. Thus,short firing times are necessary to prevent this hydrogen from‘out-gassing’ from the wafer. It is best that the hydrogen is capturedand retained within the bulk material (especially in the case ofmulti-crystalline material).

The back surface typically is fully covered by an Aluminum-based paste,while the front or top surface is screen printed with a fine network ofsilver-based lines connected to larger buss conductors to “collect” theelectrons generated within the depleted region of the underlying dopedSi emitter or near the surface. At the same time, the highest possibleopen area is left uncovered for the conversion of light intoelectricity. After these pastes have been dried, they are “co-fired”.The back surface Aluminum paste forms alloys while the front surfacepaste is sintered at high speed and at high temperature in conveyor-typemetallization furnaces to form smooth, low ohmic resistance conductorson the front surface of the solar cell.

The instant invention is directed to improved diffusion firing furnacesand diffusion processes. Currently available IR conveyor furnaces forsuch diffusion firing processes have a long heating chamber in which aplurality of IR lamps are substantially evenly spaced apart (typically1.5″ apart) both above and below the wafer transport system (wire meshbelt or ceramic roller conveyor). The heating zone is insulated from theoutside environment with various forms of insulation, compressedinsulating fiber board being the most common. The infra-red (IR) lampsincrease the temperature of the incoming silicon wafers to approximately700° C. to 950° C. This temperature is held for the 30-minute durationof the diffusion process, after which the wafers are cooled andtransferred to the next downstream process operation and equipment.

Currently available diffusion furnaces typically employ one of two typesof wafer transport systems: 1) a plurality of static (not-longitudinallymoving), solid ceramic, rotating rollers; or 2) active (longitudinallymoving) wire mesh belts, to convey the wafers through the furnace firingzone. Static, ceramic rotating-roller furnaces currently are preferredin order to minimize or prevent metallic contamination of the backsurface of the wafers.

A typical conventional diffusion furnace is on the order of 400″ long,having 160, 36″-wide IR lamps placed above the rollers, with from100-160 placed below. The total mass of the conveyor rollers is on theorder of 800 lbs, and is classified as a high-mass conveyor system.

In such high-mass, static, solid, rotating roller conveyor furnaces, theIR lamps take substantial time to bring the furnace chamber up todiffusion temperature in the range of 700° C. to 950° C. The theory ofoperation apparently is that the heated roller mass helps keep thefurnace at a more even temperature throughout, as a result of thethermal reserve provided by a large, hot mass having a substantial heatcapacity. Such furnace systems are touted as being able to compensate,in the short term, for failure of one or several IR lamps, if spreadthroughout the furnace, since the hot rollers continue to provide heatvia conduction and convection. The IR lamps below the rollers maintainthe rollers hot, and the contact of the wafers with the rollers helptransfer heat to the wafers by thermal contact conduction. Since therollers at the entrance and exit are not heated by the same number oflamps as those in the center of the furnace, there is a thermal profileof the conveyor, rising at the entrance and descending at the exit.

As the demand for solar cells increases, the rates of production mustincrease, either by process improvements or adding furnaces intoservice. With respect to adding furnaces, conventional furnaces have alarge footprint and the diffusion process is very slow. In large part,because of the mass of ceramic in the furnace that provides thermalenergy, the IR lamps are run at from about 15-20% of maximum power.Running them at greater power levels would easily raise the temperaturehigher than needed for diffusion, and approach failure of metalliccomponents (e.g., in the roller drive elements secured to the ends ofthe rollers). Accordingly, the “soak” period to accomplish diffusion islong—on the order of 20-30 minutes. Thus, since the furnaces are large,adding furnaces requires increased capital outlay, for buildings, thefurnaces themselves, and related service facilities.

Thus, the need for faster production and greater throughput, whilecurbing facility capital outlay, is not being met by the current stateof the art solid, rotating ceramic roller conveyor furnaces. In order tocompensate, furnaces have been made laterally wider, so that multiplelines of wafers can be processed in each furnace zone. This in turnrequires longer, more expensive lamps which typically experience asubstantially shorter mean time to failure, thus significantlyincreasing operating costs.

Since there are dimensional and IR lamp cost constraints, increasinglamp density in the furnace is not generally a feasible solution.Likewise, increasing the power to the lamps is not currently feasiblebecause higher output can result in overheating of the lamp elements, asa result of the thermal mass of the furnace, principally in the highmass solid ceramic roller conveyor system. Overheating particularlyaffects the external quartz tubes of the lamps. Most furnaces arethermocouple controlled. Since the IR lamps are placed side by side, onthe order of 1.25″ apart, each lamp heats lamps adjacent to it. When thethermocouples detect temperatures approaching the selected diffusiontemperature set point in the 700-950° C. range, they automatically cutback power to the lamps by an amount that depends on the thermal mass ofthe transport system (rollers or metal mesh belts). This lower powerdensity is accompanied by substantial changes in the spectral output ofthe IR lamp emissions (hence a lower light flux and energy output). Inturn, this reduced light flux results in the need to slow down theconveyor belt speed or lengthen the furnace (while maintaining theoriginal belt speed), thus slowing processing. Overheating of lamps,e.g., due to thermocouple delay or failure, can cause the lamps todeform, sag and eventually fail. Lamp deformation also affectsuniformity of IR output delivered to the wafers.

Accordingly, there is an unmet need in the diffusion furnace anddiffusion firing process art to significantly improve net effective useof firing zone(s), to provide better control and thermal profilesthroughout the entire furnace, to permit improved utilization of firingenergy, to improve the speed and uniformity of the diffusion process, toreduce furnace size while retaining or improving throughput, andaccomplishing these goals on a reduced furnace footprint, and lowerenergy, operating and maintenance costs.

The Invention SUMMARY, INCLUDING OBJECTS AND ADVANTAGES

These and other unmet needs in the art are met by the invention which isdirected to a multi-zone solar cell diffusion furnace having an ultralow-mass, active transport system for conveying wafers through aplurality of heating and cooling zones, including at least one InletBaffle Zone, a diffusion Firing Zone, followed downstream by a SoakingZone, and one or more Cooling zone(s), for front and/or back sidediffusion of P or/and B dopants to form p-n junction and/or back contactlayers in the wafer substrate.

As disclosed herein by way of example, the inventive transport system isillustrated in two alternative embodiments: A) a band/pin drive system;and B) a roller chain/sprocket drive system, the latter being thepresently preferred embodiment. In both embodiments, the wafers aresupported during longitudinal transit through the processing zones onnon-rotating small diameter hollow refractory tubes carried onsuspension wires or rods spanning the width of the transport system. Inthe band/pin drive system the opposed ends of the wires are carried bypylons formed from, or attached to, the drive bands. In the rollerchain/sprocket drive system the wire ends are journaled in hollow tubelink pivot pins.

The inventive ultra low-mass active transport system is applicable to awide range of furnace configurations, numbers of process zones, andtype, positioning and numbers of heating elements. The heating elementsemployed in the inventive furnace in combination with the ultra low masstransport system are selected to provide thermal radiation or IR fluxoutput suitable for the process, P diffusion, B diffusion or both, andare shown herein by way of example as IR lamp elements, resistance-typeradiative/re-radiative elements, and a combination of both. For P-onlydiffusion, particularly in narrow width furnaces (typically less thanabout 24″ wide processing conveyor) IR lamps may be used, and it ispreferred to employ Hi-Intensity IR Lamp isolation modules, describedbelow in more detail. For higher temperature processing, SiC rod-typeresistance elements, optionally shielded within ceramic or stabilizedSiC tubes are employed.

Combinations of heating elements may be used; for example, free-standing(non-enclosed) IR lamps followed by High-Intensity IR lamp (enclosed)isolation module(s) may be used for ramp-up to near processingtemperatures, followed by SiC resistance elements in the peak firingzone and in the soaking zone. The use of the High-Intensity IR lampisolation modules have the advantage of providing short wave lengths,high flux IR photo-conditioning which promotes faster diffusion. In thedisclosure herein, where there is reference to High-Intensity IR lamps(HI-IR), it should be understood that SiC radiative/reradiative heatingelements may be employed, the HI-IR lamp elements disclosure being byway of example only.

The transport of the advanced material solar cell wafers, such assilicon, selenium, germanium or gallium-based solar cell wafers, isimplemented in the furnace zones by use of an ultra-low mass, active(longitudinally moving), shielded transport system comprising two ormore continuous loops of laterally spaced-apart, transport elements,comprising narrow width “belts” carrying light weight, small diameter,non-rotating refractory tubes suspended on wires strung between thebelts on each side of the wafer processing travel path. The refractorytubes are thin-wall, rigid ceramic or vitreous material, preferablyselected from at least one of alumina, silica and zirconia.

The transport system “belts” are implemented in a number of exemplaryembodiments, a first being laterally spaced-apart metal, horizontallyoriented, flat bands or ribbons, each having multiple verticallyextending pylons that are longitudinally spaced-apart along the belts.The pylons carry wires on which the refractory tubes are threaded. Thewires extend laterally across the wafer travel path between the matchedpairs of pylons, one on each band. In a second embodiment, each belt isa roller chain, e.g., a robust bicycle-type chain, having hollow tubesin place of solid link pieces. The refractory tube suspension wires arethreaded through, and supported at their ends in the chain link tubes.In both embodiments, the transport elements, or “belts”, are drivensynchronously by a drive system, described in detail below. Keeping thebelts synchronized in their movement keeps the wires carrying therefractory tubes parallel to each other and straight, that is,orthogonal to the direction of wafer travel along the processing path.

In both embodiments, the wafers are carried sequentially through theseveral zones of the diffusion furnace while resting on annularstandoffs spaced along the refractory tubes, which results in lesscontamination. The standoffs may have a wide range of external profileconfigurations, e.g., conical, rounded (donut-shaped), vertical knifeedge, slanted, conical, square top fin, rib, and the like. Therefractory material is preferably selected from a high temperatureceramic or vitreous material that can be precisely configured bycasting, dry pressing, extrusion or machining, and preferably includesat least one of silica (including silica glasses), alumina and zirconia.

The inventive diffusion furnace in both embodiments typically has anumber of zones: an Inlet Baffle Zone that raises the temperature fromRoom Temperature to 500° C. and is on the order of about 20-30 cm long;a firing Zone that raises the temperature to the diffusion firing setpoint in the range of from about 900-1100° C. and is typically about30-100 cm long; a Soaking Zone that maintains the temperature at about900-1100° C. and is typically 120-200 cm long (e.g., 4, 40 cm modules);and one or more Cooling Zones that brings the temperature back down toroom temperature. The CZs are typically two or more zones, a first of30-50 cm in length that drops the temperature from 900-1100° C. to 500°C., and a second on the order of 60-90 cm in length that drops thetemperature from 500° C. to Room Temperature. The second Cooling Zonemay be external of the furnace shell. Of course, one skilled in the artwill readily appreciate that the length of the zones depends on processtransport system width, rate of travel, and the heating profile andprocess schedule, including power to lamps, among others. Accordingly,the lengths of zones listed are non-limiting examples.

The firing zone(s) configurations are not critical to the inventiveapparatus or method of operation, there being a wide range ofarrangement of the High-Intensity IR lamp or resistive heating elements.For example, in a first embodiment, for narrow width furnaces (narrowtrans-port systems) or for top-side P doping only, all heating may beimplemented using HI-IR radiant flux lamps in all zones. In such processapplications it is preferred to use at least one HI-IR zone employing IRlamp isolation modules. In such embodiments the other zones, includingthe ramp-up buffer zone, and the soaking zone(s) may be free-standing IRlamp heated.

In a second embodiment, for wide process path furnaces on the order of1-2 meter-wide transport systems or for processes of doping with B, orfront side with P and back side with B, where temperatures are above950° C., e.g. in the range of 1000-1100° C., the firing heating elementsare presently preferred to be resistive radiant types, such as SiC rodsor coiled elements, optionally housed in protective re-radiant (opaque)ceramic-type tubes.

In either heating element configuration embodiment the heating elementsmay be mixed, that is, free-standing IR lamps, HI-IR radiant fluxisolation modules, and resistance thermal radiant/re-radiant elementsmay be used. For example, the input ramp-up (baffle) zone may befree-standing IR lamps, followed by a first HI-IR flux isolation module,followed by high temperature resistive radiant/re-radiant elements in asecond firing zone and in the soaking zone(s). The soaking zone(s) maybe simply an extension of the firing zone. That is, the name(s) of thezones is not the governing factor, the selection of type, position andnumber of elements, above and below the transport system beingimplemented to achieve the desired firing temperatures and schedule forprocessing be it P diffusion, B diffusion, or both.

The wafers do not touch wire mesh belts or ceramic rollers, beingsupported on stand-offs such that there is no metal contamination, nohot spots are developed in the wafers, and the wafers do not wander toone side or the other, as in conventional roller conveyor systems. Inaddition, the diffusion process of this invention is a high radiant fluxdriven process, rather than being a thermally conductive, longwavelength process.

In the first wafer transport system embodiment, the transport systemside edge bands include longitudinally evenly-spaced drive holes. Eachband is configured in an endless loop comprising a transport section(forward motion through the processing Zones) and a return section. Theband loops are driven synchronously by one or more pin drive roller(s)at the outlet end of the furnace. Each band is tensioned individually bya tensioning system disposed in the return path. This drive arrangementresults in the transport (top) segment bands being pulled through thefurnace process zones. The transport system operates at very lowtension, on the order of 20 psi or lower, preferably below about 5 psi.This results in less stretching of the belt, long life, low-frictionduring band transport, less wear, substantially no metalliccontamination, and overall lower energy requirements for furnaceoperation.

The bands are typically high temperature resistant metal, such as amember of the family of austenitic nickel-based superalloys, thecurrently preferred being Nichrome brand, 80/20 nickel/chromium alloy.Other band materials include titanium, Inconel, e.g. type 600 Inconel,or other high temperature alloys. The bands are on the order of 1″ wideand 0.020″ thick. The pins of the drive wheels are tapered and the bandpin drive holes are 1/86″- 3/16″ in diameter. The pin drive holestypically spaced 1″-3″ apart along the bands, depending on the number ofpins and diameter of the pin drive roller. The bands slide in shieldedchannels on each side of the furnace zones, the channel members beingconstructed of alumina, silica, quartz or other high-temperature, lowfriction, ceramic material. The band channels are configured to provideat least partial shielding from the firing elements to minimize theheating of the bands. Optionally, the bands are cooled by directingambient, or cooled, compressed air onto the bands, preferably frombelow, and channels or ports are provided on the exterior lateral sideof the furnace zone module above the bands to exhaust the cooling air.

In the second transport embodiment, roller chains are used in place ofbands, and the vertical pylons are not needed. Rather, the suspensionwires are journaled at each end in a tubular sleeve of the chain. As inthe band embodiment, each chain slides in a groove or channel in, orstraddles a guide ridge on slider blocks of low friction, hightemperature ceramic material. The slider blocks may be continuous orspaced to bridge the juncture of adjacent zones, and also serve asguides to maintain the linear tracking of the chain. The chains aredriven by a sprocket drive system (two laterally-spaced drive sprocketson a common drive shaft) below the outlet end of the diffusion furnace,downstream of redirecting idler sprockets that turn the chains from thehorizontal process path downward to the drive sprockets. The chains thenpass backward over a spring-biased tensioning system on the return path.At the front end of the furnace laterally spaced second redirectingidler sprockets turn the chains upward to the inlet idler sprockets,which redirect the chains back onto the process path, completing thecircuit. Optionally, the chains may be cooled in the cooling section ofthe furnace and/or on the return path, preferably by induced draft air,or compressed air.

With respect to the high intensity IR flux heating embodiment, a HI-IRzone quickly (within about 2 seconds) ramps the temperature to thediffusion process set point within the range of from about 700° C. toabout 950° C., while sensitizing the dopant-coated wafer surface byphoto-irradiating it with high intensity short wave-length IR radiantflux. The high intensity short wavelength IR radiation photo-conditionsthe wafer surface for faster diffusion. As compared to processes whichare long-wavelength radiation processes, this embodiment utilizes shortwavelength-IR radiant flux in the diffusion process.

As compared to currently available commercial diffusion furnaces, theinventive embodiment employing one or more zones of high intensity IRflux involves a step of photo-conditioning the wafer surface, whichshortens the diffusion processing time by half or more, with theresulting throughput being double or greater. By way of example, inoperation, the inventive furnace can complete diffusion processing in 6minutes, as compared to the currently conventional process in rates of12-14 minutes. As a result, the throughput is double or greater. Inaddition, the resulting resistivity of the cell p-n junction layer isnot only more uniform across the wafer, and consistent from wafer towafer, but also lies in the “sweet spot” of between 45-100 ohms/cm².

It is an important aspect of the inventive rapid diffusion process thatwhere IR lamps are employed, they are operated at a substantially higherpower than conventional furnaces, as a % of maximum lamp-rated power.For control of power to the lamps, several alternative control modes maybe employed. In a first embodiment, the controller employs anexperience-based algorithm to adjust power to the lamps in each zone andtop vs. bottom, in accord with a preset desired temperature, by voltagecontrol only. In a second embodiment, the temperature in each respectivezone is monitored by thermocouples, and the voltage to the lamps isadjusted via a feed-back loop control algorithm. In addition, thevoltage is monitored to insure excess voltage is not supplied to thelamps in the event of thermocouple failure.

In conventional static, rotating-roller furnaces, lamps are operated atfrom 5-20% power, once the zone is at temperature, due to the inabilityto remove significant amounts of heat from each specific zone, since therollers are static and have high thermal mass. This means the lampoutput must be reduced to maintenance levels, thereby reducing theradiant flux. Lower power to the lamps also results in a significantshift of radiant output, to a mix or suite of longer wavelengths. Thecombined lower radiant flux and longer wavelength mix results in littleif any photo-conditioning of the wafers. Thus, current static,rotating-roller, thermal diffusion furnaces are characterized by lowerflux and longer wavelength without effective photo-conditioning.

In conventional metal mesh belt furnaces, heat removal is not as much aproblem. The upstream zones can operate at higher power due the need toheat the belt, but downstream zones will be operated at lower powerlevels due the belt already being at temperature. The major problem withmetal belts, however, is metallic ion contamination of the wafers sincethey rest directly on the metal mesh. Even using ceramic bead-coating onthe metal mesh belts, a metal ion cloud boils still off the metal meshof the belts, adversely affecting the cell layer chemistry.

Attempts have been made to coat the bottom of the wafers with a P dopantto reduce the metal contamination from metal mesh belts. However, thisresults in creating a p-n junction layer on the bottom of the wafer. Inturn, this requires an extra process step of etching-off that bottom p-njunction layer. That etching step is typically a batch process, whichtakes additional time, slowing production. This back etching problem wasa major incentive to development of solid, high mass static, ceramic,rotating-roller systems.

These problems are addressed and solved by the inventive low massceramic tube transport system. First, the metallic transport components,bands or chains, are placed at the sides of the furnace zones and thesecomponents are configured to be shielded from element radiation [heat],which prolongs component life. Second, the transport cross wires arecompletely shielded within low-mass ceramic tubes, which arenon-rotating ceramic supports in minimal contact with the wafers. Theshielded driven band or chain elements, in combination with wafersupport wires being shielded by ceramic tubes, assures a cleanatmosphere, making the firing zone substantially free of metallic ioncontamination. Third, the ceramic tubes are, in total, much lower massthan rollers and, while non-rotating, are also active, i.e., moving intoand out of the furnace, so there is no large, static thermal massrequiring power scale-back. Further, since the ceramic tubes aresuspended on wires, if the ceramic tube cracks vertically, it isretained on the wire and the furnace need not be stopped immediately forreplacement. In contrast, in solid roller furnaces, when a rollerbreaks, the furnace must be stopped. Finally, the wafers are not incontact with the ceramic tubes, being raised above them on stand-offswhich preferably are configured to only support the wafers at the edges.

In contrast to the 5-20% operating power level of the above-describedconventional, currently available thermal diffusion furnaces, in theinventive process embodiments employing IR lamps, they are operated at40-70% power, or greater. This is made possible by use of HI-IR lampisolation modules in combination with an ultra-low mass active transportsystem, so that there is little static conveyor thermal-masscontribution to the process operation.

As a result of operating at twice or greater power, the lamp IR flux inthe inventive system using HI-IR modules is substantially higher, andthe peak is kept in the short IR range of below about 1.4 microns. TheIR wavelength peak of the inventive process is at about 1.25 microns.The relative intensity developed by the IR lamps in accord with theinventive process is some 4-5 times greater than in conventional thermaldiffusion furnaces described above.

In addition to the conventional furnace operations having far lower IRintensity, the peak in those processes is broader, shallower (lowerintensity) and shifted to longer wavelengths, having a maximum at about1.75 microns, but having substantial contribution at the longwavelengths in excess of 3.0 microns. Thus, the lower power operation ofconventional IR lamp furnaces shifts the spectral profile to lowerenergy wavelengths, and reinforces their thermal-conduction operationalmodel. In contrast, the high power, high intensity, short wavelength IRspectral profile of the lamps used in the inventive process confirm theradiant flux nature.

As noted, the inventive furnace in some embodiments employ HI-IR lampisolation modules in the ramp or/and HI-IR zone(s). These modulescomprise insulating reflector elements having parallel transverse (tothe direction of transport) channels, centered in each of which channelsis/are one or more IR lamp(s). The channels are covered with anIR-transparent transmission window, such as quartz, Vicor, Pyrex, Robax,other high temperature glass, synthetic sapphire, and the like. The highintensity, multi-IR lamp, isolation modules are disposed facing eachother and spaced apart, one module above the furnace conveyor transportsystem, and optionally one module below, to define the selected IRlamp-heated processing zone between them, from which zone the modulelamps and cooling air channels are isolated.

The IR-isolation of the lamps, implemented by the cooling/reflectorchannels and window module, prevents adjacent lamps from heating eachother. The channels have a wide range of cross-sectional geometries,including square, rectangular, triangular, semi-circular, parabolic, orthey form partial pentagonal, hexagonal, octagonal or ellipsoidal forms.The channel geometry is selected to direct the IR radiant energy towardthe wafer product traversing the zone on the moving furnace conveyorbelt, rather than heating adjacent lamps by direct radiation. Thechannels are open at their opposite ends for inlet, or/and exhaust ofcooling gas flow. Cooling gas is introduced at least at one end of eachchannel via a manifold, and is exhausted at the other end, or mediallyof the ends.

The cavity, or channel-configured, reflector element is placed in tightcontact with the IR transmission plate “window” to form the isolationcooling channels which maintain the cooling air in close proximity tothe lamps for good heat transfer. The transmission plate keeps highpressure/high velocity lamp-cooling air/gas from entering and disturbingthe process region through which the transport system carrying siliconwafers pass, while at the same time permitting large quantities ofcooling gases to be used to maintain adequate cooling of the lamp quartzas well as the glass/quartz transmission plate. This is important, asmost high temperature glasses such as “Robax” are only usable to about970° C. and quartz starts softening at 1000° C. Therefore by isolationand cooling, this invention permits operation of the IR lamps at powerlevels that would normally cause the lamp casings to soften and warp,which otherwise results in shortened life spans.

This isolation geometry plus cooling of the inventive IR lamp isolationmodules permits increasing the power to the lamps from the currentstandard of 15-20% power density, to the range of 40-70% or higher. Thisresults in increase in the heating rate in the ramp-up and HI-IR zone(s)to from about 30° C./sec (conventional furnace) to about 80-150° C./sec,using conventional 100 watt/inch IR lamps. That effectively increasesthe heating rate by 3×-5× greater than that of conventional furnaces,yet without resulting in lamp turn down, shut down or deformation. Inaddition, the inventive lamp isolation/cooling system permits increasingthe conveyor belt speed. That results in a substantial increase inyield, or permits shortening the length of the furnace (for the sameyield) which reduces the furnace footprint.

By way of example only, whereas currently available conveyor diffusionfurnaces need 12-25 minutes at temperature to accomplish the diffusionoperation, the inventive high-IR flux system can complete the process in4-10 minutes. The current furnaces are wide (24-36″ wide belt) and long(300-400″ long), operate at conveyor speeds of about 30-40″/minute. Incontrast, the inventive furnace belt is 15-24″ wide, the furnace length200-300″ long, and the transport system rate is in the range of about60-100″/minute for the same productivity (on average, 1200 wafers/hr).From 2-6 lamps are used in each HI-IR isolation module, which has alength of from about 6″ to about 12″. That is, for exemplary front-sideP diffusion, a furnace in accord with the principles of the inventioncan have a narrower footprint, yet by virtue of greater rate ofoperation, has the same throughput as wider furnaces.

In order to maintain high flux via high lamp power density in theSoaking Zone for P diffusion, to achieve high speed processing, heat isremoved from the zone. Heat removal is implemented in the inventivefurnace by the active ceramic tube transport system (a very minorpercentage) and by introduction of filtered cooling air (the majorpercentage of heat removal). The cooling air flow is from top to bottomin the Soaking Zone to suppress deposit of particles on the top surfaceof the wafers and to remove the particles. Thus, in contrast to highmass, static, ceramic, rotating-roller conveyor systems wherein theprocessing approach is to not remove heat from the processing zone(s),in the inventive system and process, heat is removed in order to be ableto maintain high power density, and permit high flux, short wavelengthIR wafer photo-conditioning, thus speeding the process. Recall that heatis also removed from the HI-IR Zone by the cooling air flow through theisolation lamp module channels. While heat removal seemscounter-intuitive, the high flux, short wavelength IR more thancompensates for heat removal.

Power to heating elements, whether IR lamp or resistive (SiC) radiantelements, top and bottom, is adjusted independently or in groups toachieve precise temperature gradient control in each zone. Temperaturecontrol may be effected using either thermo-couple-based temperatureregulation, voltage-controlled power regulation, or hybrid systems,employing a PID controller as described above. Regulation of power tothe lamps is preferably voltage-controlled, as it permits ease ofmaintaining stable lamp power for the preferred high values of IRintensity (radiant flux), and constant spectral output at all times. Inaddition, running the lamps at higher power density increases the IRflux and also provides a better spectral range, with the peak in theproper position.

Accordingly, the inventive diffusion furnace employing ultra low masstransport system permits rapid diffusion firing, in which the resistanceelement heating or/and IR lamp radiant flux heating with spectralprofile peaking at 1.25 microns throughout the entire process, to drivediffusion of P or/and B atoms into the Si (or other advanced material)matrix to form the p-n junction or/and B-diffused back contact surfacelayers. The heat is directed to the wafers, not the transport system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail with reference to thedrawings, in which:

FIG. 1 is a schematic of a first embodiment of the inventive diffusionfurnace showing in side elevation the Inlet Transition Zone, a BaffleZone or/and Ramp-Up Zone, a Firing Zone, a Soaking Zone, a Cooling Zoneand a low mass drive system;

FIG. 2 is an isometric view of the inventive low mass transport system,in this embodiment a band-type transport, as mounted in several of theheating zones of a diffusion furnace with the front, inlet end of thefurnace being on the right, and also showing wafers on the aluminasupport tubes;

FIG. 3A is an enlarged view in isometric of the band transportembodiment of FIG. 2 in the inlet baffle zone and a HI-IR lamp zoneshowing transport of two exemplary wafers through the furnace;

FIG. 3B is an isometric view from underneath the lower section showingthe exhaust manifold for a Soaking Zone;

FIG. 4A is an enlarged isometric view of the band transport exploded outof FIG. 3A to show the details;

FIG. 4B is an enlarged isometric view of the pylon assembly and the bentwire tip retainer for the band transport embodiment;

FIG. 5 is a schematic of the inventive furnace in side elevationoverlying a graphic profile of the temperature developed in thecorresponding zones vs time;

FIG. 6 graphically illustrates that spectral output is the key to IRlamp-heated diffusion process speed, with FIG. 6A showing the SpectralOutput in the inventive low mass transport system in terms of relativeintensity vs. wavelength, and FIG. 6B shows the comparative SpectralOutput in a high mass, solid ceramic roller system in terms of the samerelative intensity vs. wavelength;

FIG. 7 is a schematic of a second embodiment of the inventive diffusionfurnace showing the several processing Zones and chain drive wafertransport system;

FIG. 8 is an enlarged view in isometric of the chain transport system inthe furnace processing zone(s) showing transport of two exemplary wafersthrough the furnace;

FIG. 9 is an enlarged isometric of details of the chain drive showinghow the suspension wires and ceramic tubes are journaled in hollow chaintubes;

FIG. 10A is a vertical section view through the line 10A/B-10A/B of FIG.9 showing a first embodiment of a slider plate guide for the chaindrive, in this embodiment a channel;

FIG. 10B is vertical section view through the line 10A/B-10A/B of FIG. 9showing a second embodiment of a slider plate guide for the chain drive,in this embodiment a rib;

FIG. 11A is a vertical section view through the suspension wire andalumina tube showing a third embodiment of the wafer stand-off; and

FIG. 11B is a vertical section view through the suspension wore andalumina tube showing a fourth embodiment of the wafer stand-offconfiguration.

DETAILED DESCRIPTION, INCLUDING THE BEST MODES OF CARRYING OUT THEINVENTION

The following detailed description illustrates the invention by way ofexample, not by way of limitation of the scope, equivalents orprinciples of the invention. This description will clearly enable oneskilled in the art to make and use the invention, and describes severalembodiments, adaptations, variations, alternatives and uses of theinvention, including what is presently believed to be the best modes ofcarrying out the invention.

In this regard, the invention is illustrated in the several figures, andis of sufficient complexity that the many parts, interrelationships, andsub-combinations thereof simply cannot be fully illustrated in a singlepatent-type drawing. For clarity and conciseness, several of thedrawings show in schematic, or omit, parts that are not essential inthat drawing to a description of a particular feature, aspect orprinciple of the invention being disclosed. Thus, the best modeembodiment of one feature may be shown in one drawing, and the best modeof another feature will be called out in another drawing.

All publications, patents and applications cited in this specificationare herein incorporated by reference as if each individual publication,patent or application had been expressly stated to be incorporated byreference.

FIG. 1 is a schematic diagram of the inventive diffusion furnace 10which comprises a framework and housing 12 having a lower section 14 andan upper section 16, the framework 12 optionally being configured withlinear actuator lifts spaced along the exterior (see FIGS. 2 and 3) toraise the upper section 16 relative to the lower section to permitservicing of the lamp assemblies and the transport system. The furnace10 comprises a plurality of sections or zones as follows, from the entryor front end 18 (on the left in this figure) to the outlet or back,downstream end 20 (on the right):

-   -   IT, the Inlet Transfer end 18 from an upstream dopant applicator        unit (not shown);    -   B-1, Inlet Baffle Zone employing one or more compressed air        knife assemblies 22; optionally the B-1 Zone comprises a Ramp-Up        zone containing one or more heating elements to raise the wafer        temperature from ambient to on the order of 500° C.;    -   FZ, the initial Firing Zone for raising the wafer temperature to        the diffusion temperature in the range of from about 900° C. to        about 1100° C. depending on whether P, B or both are diffused;        the FZ can further be subdivided into two or more zones, e.g.;    -   HI-IR Zone, High Intensity IR lamp array, preferably upper        or/and lower isolation reflector lamp assemblies 24-U, 24-L for        temperatures up to about 950° C., followed by;    -   HTZ, High Temperature Zone, heated by resistance radiant (SiC)        elements 27 for temperatures up to 1100° C. depending on whether        P, B or both are diffused;    -   S, Soaking Zone, having spaced upper and lower IR lamps or        resistance elements, 26-U, 26-L;    -   B-2, Exit Baffle Zone with air knife assemblies, 22;    -   C, Cooling Zone, typically with no element or IR lamps; and    -   OT, Outlet Transfer Zone for transfer of the diffusion fired        wafers to the processing equipment for screen printing collector        fingers and busses on the front side and the back contact layers        on the back side (not shown), which is followed by firing to        form the ohmic contacts. The Outlet Transfer Zone may optionally        include upper or/and lower air knife assemblies adjacent the        exit of the furnace (on the right).

Upstream and downstream of the inventive furnace are low temperaturetransfer belts 28-U (Upstream) and 28-D (Downstream), typically,elastomeric (O-ring) belts on the upstream end from a doping coatingapplicator assembly, and wire mesh belts on the downstream end to ananti-reflection layer applicator, a screen printer and thence to ametallization furnace. These low temperature transfer belts 28-U and28-D interface with the drive system 30 of the inventive furnace 10.

The inventive furnace low-mass drive system 30, in a first embodiment,comprises a pin drive roller 32, driven by a motor 34 and chain or belt36 disposed at the outlet (right) end of the furnace, wafer transportband assemblies 38, idler rollers 40 and a tension system 44 whichincludes a tension roller 42. The tension system 44 includes anautomatic tension compensator spring acting as a dashpot to assist inpreventing surges. Note that by this drive geometry, the bands 38 arepulled along the feed path, F, through the zones from left to right.This is implemented by the drive assembly 30 pulling the upper levelbands from left to right (as indicated by Arrow F—forward).

The inventive furnace includes a plurality of plenums defining the Zonesinterior of the housing 12, and a plurality of air manifolds, includinginlets and exhausts for ambient or pressurized air flow into the variousZones as shown by the arrows I (for Inlet), and E (for Exhaust), inorder to maintain the proper temperature in the respective zones. Inaddition, pressurized air is provided in the channels of the reflectorbody of the isolation lamp assemblies 24-U and 24-L (where used) to coolthe lamps disposed in the channels. The face of each of the highintensity lamp assembly 24-U and 24-L is covered with a sheet of clearquartz to seal the reflector channels from the wafers beingphoto-conditioned in the HI-IR zone. This insures the air stays in theannulus between the contoured reflector surface and the lamp to providethe necessary cooling. That is, the lamps are isolated from theprocessing zone. By way of implementing example, the air is introducedat each end of the lamp (the sides of the furnace), and exhausted out anoutlet in the middle, that is, along the vertical centerline of thefurnace. This lamp cooling permits the lamps to be run at from about 60%to 100% of maximum power, which is far higher than presently availablecompetitive units. This provides an immediate thermal boost and highintensity IR irradiation of the wafers from ambient temperature to thepeak diffusion conditioning temperature of 700° C. to 950° C. (for Pdiffusion), and the high intensity photo-conditioning.

FIG. 1 is of such scale that the solid thermal insulation material 46cannot be shown in every position, but it will be evident to one skilledin this art that the housing includes necessary configured blocks ofinsulation material. For example, in the Soaking Zone, the blocks ofinsulation include a transverse bore and a vertical inlet bore connectedto the inlet air pipe so that pressurized air is fed into the block, andair can pass evenly through the porous insulation material to provide acontrolled flow into the Soaking Zone, thereby maintaining thetemperature in the required range, while minimizing the energy costs.That is, there is a counter-flow of air (shown by arrows 48) through theinsulation into the zone which is opposite the heat flow. This resultsin the air, as it enters, picking up heat from the insulation andrecycling it into the zone. The result is a highly efficient heatexchange operation for this zone. Note the lamps 26-U and 26-L orresistive elements 27 are shown staggered to provide an even heatingand/or IR photo field, but they are optionally arranged not staggered,that is, the lamps/elements can be located spaced one above another in acommon vertical plane.

Turning now to the low mass transport system 30, FIG. 2 shows inisometric view the furnace 12 with the outer panels and insulationremoved to show the frame of the lower section 14. The inlet end 18 isat the lower right, and the outlet end 20 is at the upper left. Theinlet transfer belt 28-U and outlet transfer belt 28-D are not shown forclarity. Also for clarity, the brackets and lifters 50 are shown at thefour corners of the furnace side rails 52; these function to lift theupper section 16 (not shown) from the lower section 14 (shown) for easeof inspection, adjustment, maintenance and repair/replacement of parts,e.g., lamps, resistive heating elements, transport system elements, andthe like. The lower section 14 includes side rails 52, sidewallinsulation blocks 46-S, and internal zone divider blocks of insulation54. The forward insulation block at the front end of the Baffle Zone B-1and dividing B-1 from the High Intensity IR zone HI-IR, and at the aftend of the cooling zone C are not shown in order to not obscure detailsof the band 38.

FIG. 1 also shows the insulation blocks 56 forming the floor of theseveral zones. These floor insulation blocks 56 typically are providedwith apertures, here slots 88, which together with a plenum below thefloor (not shown) permit extraction of hot exhaust gases by an ID fan(not shown). This airflow removes heat from the individual zones,permitting the elements (lamps, SiC rods or coils) to be run at higheroutput, and also provides for contaminant extraction since the airflowis from top to bottom. This hot gas/airflow pattern results in thefurnace zones having reduced contaminant levels, and therefore a cleanerproduct.

Apertures 58 for installation of the resistive elements 27 or/and highintensity lamp tubes 26 (only one of each being shown for clarity) andconnection of the electrical leads are shown in the facing side rail 52and the far sidewall insulation blocks 46-S, the elements/lamps spanningthe width of the furnace zones. The compressed air plenum 60 for thelower High Intensity IR lamp zone 24-L feeds compressed air via line 62into the annulus between the reflector channels and the lamps, and isexhausted below to the exterior, or into the adjacent, downstreamSoaking zone, as needed.

For clarity, only a portion of the low mass, active transfer belt system30 is shown at the right end. The side bands 38 engage pins in thespaced idler rollers 40 of the drive system 30. Just inside the Coolingzone, C, the idler roller 40 and the drive roller 32 below it can beseen. Two wafers, W-1 and W-2 are shown at the inlet end (on the right)placed on the transverse alumina tubes 64 in the position of transferthrough the furnace.

FIG. 3A is an enlarged isometric view showing the inlet end 18 of theinventive furnace 10 of FIG. 2 showing a portion of the band transportsystem 30 carrying two wafers W-1 and W-2, the numbering of parts andelements being the same as in FIGS. 1 and 2. The two spaced bands 38 ofthe transport system 30 are disposed in U-shaped channels 66 formed intothe tops of the left and right sidewall insulation blocks 46-S. Eachband includes precisely spaced holes 68 that engaged pins 90 of theidler rollers 40. Also spaced regularly along the bands are upstandingpylons 70 which carry wires 72 on which are placed ceramic tubes 64. Anexemplary wire is 0.080″ diameter Nichrome. The wafers rest on theceramic tubes. Preferably standoffs 84 are provided, here rings in thisembodiment, mounted, on or formed into, the tubes and spaced laterallyapart along the tubes. The wafers rest on the stand off rings duringtransport through the furnace 10, so that there is minimal contact ofthe wafer back surface with the transport assembly elements, seefootprint 96 in FIG. 4A.

In the event a tube 64 breaks or cracks, it will be retained by the wire72 until it can be replaced. The pylons of each band move in parallelcoordinate relationship due to the indexing of the holes 68 in the bandwith the pins 90 of the rollers, (drive and idlers), thus the aluminatubes remain transverse to the direction of wafer feed travel, as shownby the Arrow F. A plurality of photo-optical band-break sensor(s) areused, typically one at each end of each band, to quickly sense if a bandbreaks to signal the controller to stop the transport system, shut downthe lamps and sound an alarm.

FIG. 3B is an isometric from underneath the lower section 12 of thefurnace with the framework removed for clarity. The side frames 52 arespanned by the floor insulation blocks 56 having exhaust slots 88 (bestseen in FIG. 3A). Spaced from the bottom of the insulation blocks 56 isa steel panel 100, the space providing a collector plenum for the hotgases. An exhaust manifold assembly 102 is connected to the plenum(space between 56 and 100) via collars 104 on the ends of cross-pipes106. The opposite end of the cross pipes are connected to a collectorduct 108, which exhausts the hot gases out an exhaust pipe or flue pipe110. The venting may be implemented in any suitable way, e.g., by forcedair into the furnace at the top, by induced draft in the exhaust path,by natural flue (chimney) effect, or by an eductor effect, such aswherein compressed air is fed into the periphery of a venturi to form avacuum that sucks hot air out of the furnace.

FIGS. 4A and 4B should be considered together. FIG. 4A shows inisometric the low mass transport system 30, and the U-shaped channelassembly 74 in which the bands 38 of this embodiment move, isolated andexploded out of FIG. 3 to show the details of construction andoperation. FIG. 4B is a close up of a single pylon assembly of FIG. 4A.

Each band 38 is supported on a quartz slider member 74, U-shaped incross-section and having short vertical side walls. A pair of keeperstrip members 76 made of quartz, alumina, or other high temperature,fiber-type ceramic material are glued to the tops of the U arms with ahigh temperature ceramic cement, such as #1 Insa-Lute or #2 Aluseal highstrength silica or alumina cement of Sauereisen Company of Pittsburgh,Pa. The keeper strips overlap the channel sufficiently to assist inphysically keeping the band in the channel and to shield it from theheating elements in the zones, thereby keeping it cool. Optionally, thebottom of the slider members can be bored with holes and connected to aninput air manifold so that ambient compressed ambient air can be used tocool the bands.

The pylons 70 include a vertical leg 80 secured to an upright tab 78that has been formed by punch-out 92 from the band. The leg 80 and tab78 may be secured together in any suitable manner, e.g. by spot welding,riveting through coordinate tab and leg holes 94, or with screws. Theupper end of the pylon leg 80 carries one or more ears 82, a pair beingshown by way of example, each ear having a hole into which a hightemperature resistant wire or rod 72 is journaled. An alumina tube 64 isslid over and carried by the wire. The wafers, W, are carried on thealumina tubes. Preferably each tube includes a plurality of laterallyspaced stand-off members 84, here rings, on which the wafers rest. Thering members present far less surface area for the underside of thewafer to contact, as shown by the footprints 96, shown in phantom inFIG. 4A. This stand-off function keeps the wafers cleaner, and helpsconcentrate the IR flux or heat radiation onto the wafers.

The stand-off rings 84 may have a wide variety of cross-sectionconfiguration, ranging from a simple flat surface torus as shown in FIG.4A, to a tapered profile, e.g., one that is bell-curve shaped incross-sectional profile. See FIGS. 10A, 10B, 11A, and 11B for variousprofiles. Note in FIG. 4B the wire 72 is trapped between the two ears,to prevent lateral movement of the wire such that the other end (rightside) falls out of the ears of the pylon on that side. Note in FIG. 4Athat the right side of the wire extends well past the outer ear of itsrespective pylon.

The use of wire having a free end permits the wire length to expand andcontract without falling out of the holes in the ears. In addition, atleast one end of the wires includes a bent tip 86 which keeps the wirelocked in place. While the bent tip can be located beyond the outsideear, it is preferred to have the bent tip trapped between the ears, asshown. This locks the wire from lateral movement, yet is easy to insertby a simple angular insertion through the inside ear hole. The wire at0.080″ diameter fits sufficiently closely in the 0.120″ bore of a tube64 that the natural minor curves of bends in the wire prevent the tubsfrom sliding laterally. They also act as a stop, preventing the wiresfrom disengaging from the ears.

It is an important feature of the band and chain embodiments of thetransport system of this invention that they are ultra low mass, simpleto install, easy to keep clean, has few contact points on the waferswhile transporting them through the furnace, and is easy to maintain. Asto the latter point, it is an option to replace a ceramic tube while thebands are on the return portion of the cycle, without stoppingproduction. However, as seen in FIG. 4A, since there are so many wiressufficiently closely spaced to provide support for the wafers, a brokentube and/or wire normally will be left for replacement at the time of aplanned shut-down. That is, loss of a tube or wire does not seriouslyimpact the product throughput.

The ultra low mass aspect of the inventive transport system is veryimportant. In some currently available competitive IR lamp or resistanceheating element furnaces, instead of a metal mesh belt, an array ofclosely spaced, solid, precision machined, relatively large, rotatingbut static (not longitudinally moving through the furnace) ceramicrollers are used as a roller conveyor system. The wafers ride directlyon the rollers which are rotationally powered by drive sprockets orpulleys secured to the ends of each roller driven by a sinusoidal drivechain or belt. Such furnaces are available from SierraTherm,Watsonville, Calif. as its XR Series for in-line diffusion of P dopantsinto silicon wafers for solar cells. Another rotational, large ceramicroller diffusion furnace is made by TechnoFimes of Milan Italy. In thesefurnaces, the rollers statically stay in place but rotate to power thewafer movement over them through the furnace. That is the opposite ofthe inventive system, in which the small ceramic tubes do not rotate,but move through the furnace. However, with respect to the wafers, theceramic tubes in the inventive furnace are static, that is, they do notmove relative to the wafers.

A first serious problem with such roller-type furnaces is the total sideedge to side edge, and front edge to back edge contact of the wafer backsurface with the entire surface of each and every roller, thus promotingcontamination. In addition, the energy required to heat the high mass ofthe rollers, and to maintain them at temperature, is far higher than inthe inventive system. In the inventive band transport system, theceramic tubes are smaller than the rollers of SierraTherm orTechnoFimes, being typically ¼″ diameter vs. 1.5″ diameter. Theinventive transport uses 0.8 oz hollow tubes having a 0.120″ bore,rather than being 3-5 lb solid rods. Thus, the mass of the inventiveconveyor system is on the order of 1/50^(th) that of the solid ceramicroller system, and the mechanics of transport are far faster, simplerand easier to maintain. To clean the inventive system only calls for asimple change out of a few tubes, rather than scrubbing, grinding,burn-off or change-out of one or more heavy solid rods in theinter-linked solid ceramic roller array of the SierraTherm orTechnoFimes furnaces. The inventive furnaces can be half the width ofsuch conventional roller furnaces and still provide a greater throughputand better yield.

Another problem with solid ceramic rod-type roller systems is thepotential for hot-spots, and thereby uneven diffusion. As the wafersmove over the rollers, the rollers are intermittently exposed directlyto the IR or resistance element radiation, so the leading edge of awafer repeatedly contacts a hotter roller as it traverses the furnace.In contrast, in the inventive furnace transport system, the wafer, fromthe very first introduction into the High Intensity IR zone shields, andnever moves relative to, the refractory tube stand-offs on which itrests. Since the wafer is spaced by the stand-off above the surface ofthe small-diameter alumina tube, there is room for hot air to circulatebetween the wafer back surface and the tube, as well as allowingindirect radiant heating by adjacent heating elements from the front andback (top and bottom sides) to provide radiant energy. Since theinventive tubes are so small, there is no shadowing of the wafers, thusproviding more even heating of the wafers. There is also no intermittentexposure to large hot rollers, and less back surface contact withsupports, which stay at uniform temperature.

Finally, in the furnace embodiments using high intensity IR lamps orisolation modules adjacent and downstream of the immediate entrance ofthe inventive furnace, the wafers are extremely rapidly heated (within afew seconds) into or near the diffusion temperature range. In thisembodiment, the wafers are photo-conditioned with high intensity shortwavelength IR radiant energy, on the order of 4-5 times or more greaterthan present roller furnaces, so that the diffusion proceeds far faster.By way of example, in the inventive furnace, the temperature gets up todiffusion temperature within seconds. More importantly, by use of theisolation modules in a HI-IR zone, and the ability to power those andSoak zone elements at higher voltage, the IR flux is higher during theprocess, and diffusion is complete in less than 6-8 minutes, as comparedto twice to three times as long with present systems.

FIGS. 5 and 6 illustrate these principles. The upper portion of FIG. 5schematically shows the inventive furnace in side elevation overlying agraphic profile of the temperature developed in the corresponding zonesvs time. The dotted curve P is the temperature profile in the inventivefurnace for P diffusion on the front side to develop the p-n junctionlayer solely by IR lamp heating. The solid curve shows that B diffusionin the inventive furnace to form the back contact layer proceeds atapproximately 200° C. higher and needs at least some zones heated withresistive elements, such as the SiC rods disclosed. Note the extremelysteep profile produced by the inventive furnace IR lamps or resistanceelements getting the wafers rapidly up to process temperature for P(and/or B) diffusion. The comparative, commercially available solidceramic roller furnaces using IR lamp heating show a curve for Pdiffusion generally following the dashed line profile, labeled “P.A.”(Prior Art) on the graph. Since the rollers are already hot in thefurnace, the lamps are automatically adjusted to run at a lower power(see FIG. 6B, below), thus resulting in a substantially andsignificantly lower temperature profile slope, and longer time to get totemperature, on the order of several minutes longer. The mostsignificant aspect of this FIG. 5 graph is that with the inventivefurnace, the diffusion is done substantially sooner, at point “DEndpoint” on the ordinate, the wafers proceeding to cooling and transferto screen printing, point “XFER” on the ordinate.

In contrast, the comparative Prior Art P-diffusion process (dashed linein FIG. 5) is continuing to heat-soak far longer at the lower powersetting as shown by the arrow pointing right on the dashed line. Theinventive high intensity IR radiant flux P diffusion process typicallytakes ½ to 1/36 the time of conventional thermal conduction process.Thus, the throughput is substantially higher with a much smaller furnace(under 300″ long and half as wide), as compared to comparable-outputconventional furnaces (400″ long by 36″ wide).

FIGS. 6A and 6B graphically illustrate that spectral output is a key toimproved IR diffusion process speed in photo conditioning, ramp-up andHI-IR zones. Spectral output of a lamp is a function of lamp power,expressed as a percentage of maximum power capacity of the lamp. FIG. 6Ashows the Spectral Output curve in the inventive low mass transportsystem in terms of relative intensity vs. wavelength. The upper curve isthe theoretical maximum, T, showing the IR peak at about 1.2 micronswith the relative intensity of about 12.5. Note the visual spectrum, VS,is to the left, shown in the dotted lines. The lower curve, labeled“Invention”, shows that in the inventive ultra-low mass transport systemusing Hi-IR lamp modules, the IR lamps can be operated with a lampvoltage control system at from about 40-100% of rated maximum, hereshown at about 40-70%, and the intensity maximum at the peak is 8.

In contrast, FIG. 6B shows the comparative Spectral Output in a highmass, solid ceramic roller system in terms of the same relativeintensity vs. wavelength. In such a comparative system, run withthermocouple-type thermal monitoring control feedback system, the lampsmust be run at about 20% power. However, there is an exponentialdrop-off in relative intensity, and the peak, labeled “PA”, is shiftedcloser to 1.75 microns, at a peak intensity of about 1.8, which is overa four-fold lower radiant flux than in the inventive process. Alsoimportant is the shift to longer wavelength, lower energy spectralprofile in the conventional systems.

Thus, in the inventive system, since the lamps are essentially onlyradiation-conditioning and heating the wafers, the HI-IR Zone lamps andthe Soaking Zone are cooled, and there is substantially no heatcontribution from a relatively huge mass of rollers, the lamps can beoperated at greater power, resulting in a 4-5-fold increase in therelative intensity. This increased IR intensity, applied sooner to thewafers, conditions them to promote the far faster diffusion of P or/andB into the advanced wafer material to form the respective junction andback contact layers. Thus, in the inventive system, the IR intensity ishigher and maintained sufficiently long for far faster processing.

FIGS. 7-10B are directed to the second embodiment of the inventive ultralow mass transport system as employing a pair of spaced chains fromwhich the wafer support wires and ceramic tubes are suspended. In FIG.7, the description above for FIG. 1 applies for the parts numbered thesame. Note that the IT and B1 zones are combined in this embodiment intoa Ramp-Up Zone wherein the wafer temperature is raised from RoomTemperature to about 500° C. to 900° C., the latter where the Ramp-UpZone includes a HI-IR isolation lamp module. This is followed by theFiring Zone which raises the temperature to the diffusion processingtemperature of about 950-1100° C., dependent on if P, or B, or both, arebeing diffused. As shown, the Firing Zone employs the disclosedexemplary resistive SiC elements. Firing set point temperature isretained in the Soaking Zone, the heating elements not being shown toprevent cluttering the drawing, but see FIGS. 1 and 5. As shown, theCooling Zone is divided into two sub-zones, CZ-1 and CZ-2. Although bothare shown within the framework of the furnace 12, the CZ-2 may beexternal, after the transfer (XFER) to belts 28D carrying the wafersdownstream for further processing, e.g., application of anti-reflectivecoating.

Note that the belts of FIG. 1 are replaced by roller chains 112 that aremoved by sprocket 114 powered by motor 34 located below the outlet endof the furnace 20. Idler rollers, in this case sprockets, 40A-40C aredisposed at the inlet, outlet and at the upstream end of the return pathR to redirect the chains in the drive loop shown. Appropriate tension isprovided by spring-biased tension and sprocket system 44, disposeddownstream of the drive 30. Guide rollers 116 and slideway blocks 74 areprovided along the chain loop to maintain the path straight. Air knives22 provide cooling compressed air over the chains in the cooling zone orexternal thereto. In addition, a tube cooler 118 may be provided in thereturn section to further cool the chain.

FIG. 8 is analogous to FIG. 4A and shows how the two, spaced-apartchains 112-L (Left side) and 112-R (Right side) ride in grooves orchannels 120 in slider blocks 74. The cross-wires 72 supporting theceramic tubes 64 have ends 122 passing through tubular link sleeves 124.The link pivots are spaced about ½″ apart (1.27 cm), and the tubularlink sleeves 124 are provided in every other link, so that the wires 72are spaced about 1″ apart (2.5 cm apart).

FIG. 9 is an enlarged detail analogous to FIG. 4B, more clearly showingthe alternate link journaling of the wires 72. The intermediate linkshave solid link pins 126. Both the link sleeves 124 and link pins 126have overlying link rollers 128 (not shown in FIG. 9; shown in FIGS. 10Aand 10B). The free ends 122 of the wires are terminated in push orthreaded nuts, or other types of fasteners 130, 140 to keep the wiresfrom falling out of the sleeves 124 (see FIGS. 10A, 10B).

FIGS. 10A and 10B show to embodiments of the slider plates 74. In FIG.10A the slider plate includes a groove or channel 120 in which the chain112 rides. The free end 122 of the wire 72 is threaded to receive alocking nut 130. An optional spacing washer 132 is shown. The standoff84 has an inverted V-shaped outer periphery, so that the wafers ride onthe circumferential edge 134. In FIG. 10B, the slider plate 74 includesa ridge 136 that can have a wide range of configurations, includingstraight or curved (inclined) side walls. The side links of the chain112 straddle and are guided by the ridge 136. In this embodiment, theslide block 74 usually does not have the outside blocks 138, the platebeing flat on both sides of the ridge, as shown by the dashed linedefining the side blocks 138. In addition, the tips 122 of the wire 72are terminated by cap or push nuts 140. The contour of the stand off isa tapered peak, being a half sine-wave in cross section in thisembodiment.

FIGS. 11A and 11B show two additional embodiments of the standoffs 84,FIG. 11A showing an asymmetric fin-shaped standoff having a slantedouter face (right of the ridge 134), and vertical or slanted side faces.FIG. 11B shows a bilaterally tapered cone shape, optionally with anannular rib 134 at the apex where the two tapered sides merge. Eitherthe rib or the long slant faces provide support for the wafer product,depending on the spacing of the standoffs 84 along the tubes 64 ascompared to the width of the products. Where the products are large, thebottom surface rests on the rib (see 96 in FIG. 4A), and where theproducts are not as wide, only the outer, lower edge of the productrests on the slanted face as shown. Note that larger (wider) wafers orproducts would rest farther up the slope or on the peripheralcircumferential rib 134. This standoff embodiment is glued to theceramic tube 64, rather than being integrally formed with the tube. Theglue used is as described above.

INDUSTRIAL APPLICABILITY

The inventive diffusion furnace of this application has wideapplicability to the solar cell manufacturing industry, namely to theprocess steps of firing solar cell wafers to cause the diffusion of Por/and B into the wafer substrate to make p-n junction layers orconductive back surface layers. The system is clearly an improvementover currently available furnaces, providing greater throughput byvirtue of substantially shorter processing time, lower energyrequirements, less process contamination of wafers, and over-all wafershaving improved performance, based on improved uniformity in the p-njunction layer and the B-doped back surface layer. Thus, the inventivesystem has the clear potential of becoming adopted as the new standardfor apparatus and methods of diffusion of dopants into the solar celladvanced material wafers.

It should be understood that various modifications within the scope ofthis invention can be made by one of ordinary skill in the art withoutdeparting from the spirit thereof and without undue experimentation. Forexample, the high intensity, soaking and cooling can have a wide rangeof designs to provide the functionalities disclosed herein. Likewise thelinear wafer transport system bands or chains may be configured in awide range of sizes, orientations and designs, or cooled or shielded ina wide range of alternative manners. This invention is therefore to bedefined by the scope of the appended claims as broadly as the prior artwill permit, and in view of the specification if need be, including afull range of current and future equivalents.

1. Continuous conveyor diffusion furnace for processing solar cellwafers, comprising in operative combination: a) a plurality of heatingand cooling zones oriented in sequence from a furnace inlet to a furnaceoutlet, said zones being disposed in abutting relationship to definethere-through a continuous longitudinal processing conveyor pathoriented in a generally horizontal plane; b) a low-mass conveyor systemfor receiving and moving solar cell wafers along said longitudinalprocessing path from said furnace inlet to said furnace outlet throughsaid zones, said conveyor system comprising: i) a plurality of spacedapart high temperature resistant metallic wires oriented transverse saidto said longitudinal processing path, said wires having a lengthdefining a useful wafer transport width of said conveyor system throughsaid furnace zones; ii) small diameter, thin walled, non-rotatingrefractory tubes suspended on said wires to provide support for saidwafers as they are transported through said furnace zones by saidconveyor system and to substantially completely shield said wafers frommetallic vapors de-gassing from said wires; iii) said refractory tubesextending at least a substantial portion of the length of said wires andpositioned on said wires to leave exposed only short opposed side endsof said wires; iv) a pair of spaced-apart transport members, onedisposed adjacent each end of said wires, each of said transport membersforming a continuous loop through said longitudinal processing path fromfurnace inlet to said furnace outlet, and thence on a return pathoutside said furnace zones back to said inlet; v) each of said transportmembers including a plurality of receiving members spaced uniformlyalong each of said transport member continuous loop, each said receivingmember is configured to removably retain said short side end of a wiredisposed suspended between said transport members across said transportwidth; and c) a drive system disposed outside said furnace zonesdisposed to engage both said transport members for synchronous movementthrough said zones as said transport members carry said plurality ofrefractory tubes and wires suspended between said receiving members, onwhich refractory tubes said wafers are transported through said zonesduring furnace operation to process them.
 2. Continuous conveyordiffusion furnace as in claim 1 wherein said transport includes at leastone of: a) loop members selected from bands and roller chains; b)wherein bands are employed, said bands include vertically extendingpylons into which said wire ends are received; and c) wherein rollerchains are employed, said roller chains include tubular pivot links,into which said wire ends are received.
 3. Continuous conveyor diffusionfurnace as in claim 2 wherein at least one of said bands and chains areguided by low-friction, high-temperature resistant support members onwhich they slide.
 4. Continuous conveyor diffusion furnace as in claim 1wherein said transport system includes at least one of: a) saidrefractory tube support members are formed of a ceramic or vitreousmaterial; b) said ceramic or vitreous material contains at least one ofsilica, zirconia and alumina; c) said refractory tubes include annularstandoffs on which the wafers rest, said annular standoffs are disposedspaced apart along the refractory tubes to provide multiple,substantially point contacts with said wafers thereby resulting in lessheat transfer and less contamination from said conveyor system; and d)when standoffs are employed, said standoffs have a wide range ofexternal configuration profiles, exemplary ones selected from conical,rounded (donut-shaped), vertical knife edge, slanted, conical, squaretop, fin, rib, and the like.
 5. Continuous conveyor diffusion furnace asin claim 1 wherein said furnace zones are heated by elements selectedfrom IR lamps, resistance radiation devices and combinations thereof. 6.Continuous conveyor diffusion furnace as in claim 5 wherein saidelements are arrayed in a plurality of process zones, including, insequence from inlet end to outlet end, zones selected from one or moreof a Baffle Zone, a Ramp-Up Zone, a High Intensity IR Zone, a FiringZone, a Soaking Zone, and a Cooling Zone.
 7. Continuous conveyordiffusion furnace as in claim 5 wherein said IR heating is provided byhigh intensity IR lamp elements disposed in at least one array selectedfrom above said wafer transport path to direct high intensity, high fluxIR irradiation downward onto a top surface of said wafers and below saidpath to direct high intensity, high flux IR irradiation upwardly betweensaid spaced conveyor system refractory tubes onto a portion of theunderside surface of said wafers, said transport members are cooled byat least one of partial shielding from said high intensity IRirradiation, and cooling with jets of substantially ambient compressedair directed onto said spaced apart transport members.
 8. Continuousconveyor diffusion furnace as in claim 6 wherein said zones are eachmodular elements having an upper half and a lower half, divided along ahorizontal plane that is generally parallel to said processing pathplane, said upper half modules being mounted in a first, upperframework, and said lower half modules being mounted in a second, lowerframe-work, and including a plurality of powered vertical liftingmembers disposed connected to said upper and lower frameworks, saidvertical lifting members permitting lifting said upper frame-workrelative to said lower framework to expose and permit access to theconveyor system transport members, wires and refractory tubes suspendedthereon for inspection, adjustment, maintenance and repair as needed. 9.Method of continuous diffusion layer firing of Photo-Voltaic (PV) solarcell wafers that have had a doping layer containing a dopant applied toat least one surface of said wafer substrate selected from a first topsurface and a second bottom surface, comprising the steps of: a)transporting a plurality of wafers sequentially through a plurality ofheating and cooling zones from a furnace inlet to a furnace outlet, saidzones being disposed in abutting relationship to define there-through acontinuous longitudinal processing conveyor path oriented in a generallyhorizontal plane and said wafers being oriented with said first surfaceoriented upward; b) supporting said wafers during said transporting stepon a low-mass conveyor system comprising small diameter non-rotatingrefractory tubes supporting said wafers, which refractory tubes aresuspended on wires spanning across the wafer processing path and whichare continuously advanced through said zones at a selected rate; and c)heating said wafers directly by at least one of IR lamp radiation andthermal resistance radiation or re-radiation directed onto said top andbottom surfaces for a time sufficient to promote diffusion of dopantfrom said doping layer into said wafer substrate material to completedevelopment of a p-n junction layer, a back contact layer or both saidlayers.
 10. Method as in claim 9 wherein at least some of said heatingis provided by High Intensity IR lamps in at least one zone powered toprovide high intensity, short wavelength infra-red radiant flux, saidlamps being operated for at least a portion of the processing time atabove about 40% maximum lamp-rated power, said irridation being ofsufficiently high flux to photo-condition a wafer surface and to raisethe temperature of said wafer to a range of from about 700° C. to about950° C. to promote diffusion of a P-containing dopant into said wafersubstrate surface.
 11. Method as in claim 9 wherein at least some ofsaid heating is provided by SiC radiative elements operated at powersufficient to raise the temperature in at least a firing zone within therange of from about 950° C. to about 1100° C. to promote diffusion of Bfrom said doping layer into said wafer substrate material to completedevelopment of a back contact layer.
 12. Method as in claim 9 whichincludes the added step of cooling said wafer in at least one coolingzone to permit handling of said wafers in subsequent downstreamprocessing to produce functional solar cell wafers.
 13. Method as inclaim 11 wherein at least some of said heating is provided by HighIntensity IR lamps in at least one zone powered to provide highintensity, short wavelength infra-red radiant flux, said lamps beingoperated for at least a portion of the processing time at above about40% maximum lamp-rated power, said irridation being of sufficiently highflux to photo-condition a wafer surface and to raise the temperature ofsaid wafer to a range of from about 500° C. to about 900° C.
 14. Methodas in claim 10 wherein the infra-red wavelength peak is maintained atabout 1.25 microns in at least one zone.
 15. Method as in claim 14wherein said infra-red lamps are provided in infra-red lamp isolationmodules for operation of said infra-red lamps in the range of from about40% to about 70% maximum lamp rated power.
 16. Method of as in claim 9wherein the dwell time in said diffusion process from inlet to outletranges from about 4 to about 10 minutes.
 17. Method as in claim 15 whichincludes the step of removal of heat from at least one soaking zone byintroduction of filtered cooling air flowing from the top of saidsoaking zone to the bottom and removal of heated air from the bottom ofsaid zone.
 18. A furnace low-mass conveyor system for receiving andmoving products along a longitudinal processing path from a furnaceinlet to a furnace outlet through a plurality of zones, said conveyorsystem comprising: a) a plurality of spaced apart high temperatureresistant metallic wires oriented trans-verse said to said longitudinalprocessing path, said wires having a length defining a useful productstransport width of said conveyor system through said furnace zones; b)small diameter, thin walled, non-rotating refractory tubes suspended onsaid wires to provide support for said wafers as they are transportedthrough said furnace zones by said conveyor system and to substantiallycompletely shield said products from metallic vapors degassing from saidwires; c) said refractory tubes extending at least a substantial portionof the length of said wires and positioned on said wires to leaveexposed only short opposed side ends of said wires; d) a pair ofspaced-apart transport members, one disposed adjacent each end of saidwires, each of said transport members forming a continuous loop throughsaid longitudinal processing path from furnace inlet to said furnaceoutlet, and thence on a return path outside said furnace zones back tosaid inlet; e) each of said transport members including a plurality ofreceiving members spaced uniformly along each of said transport membercontinuous loop, each said receiving member is configured to removablyretain said short side end of a wire disposed suspended between saidtransport members across said transport width; and f) a drive systemdisposed outside said furnace zones disposed to engage both saidtransport members for synchronous movement through said zones as saidtransport members carry said plurality of refractory tubes and wiressuspended between said receiving members, on which said products aretransported through said zones during furnace operation to process them.19. A low mass conveyor system as in claim 18 wherein said transportsystem includes at least one of: a) loop members selected from bands androller chains; b) wherein bands are employed, said bands includevertically extending pylons into which said wire ends are received; c)wherein roller chains are employed, said roller chains include tubularpivot links, into which said wire ends are received; and d) at least oneof said bands and chains are guided by low-friction, high-temperatureresistant support members on which they slide.
 20. A low mass conveyorsystem as in claim 18 wherein said transport system includes at leastone of: a) refractory tube support members formed of a ceramic orvitreous material; b) said ceramic or vitreous material contains atleast one of silica, zirconia and alumina; c) said refractory tubesinclude annular standoffs on which the products rest, said annularstandoffs are disposed spaced apart along the refractory tubes toprovide multiple, substantially point contacts with said productsthereby resulting in less heat transfer and less contamination from saidconveyor system; and d) when standoffs are employed, said standoffs havea wide range of external configuration profiles, exemplary ones selectedfrom conical, rounded (donut-shaped), vertical knife edge, slanted,conical, square top fin, rib, and the like.